From Micro Eye:
ZipOut is an attachment management suite that integrates seamlessly with Outlook. Attachments are the principal component of message traffic and ZipOut will automatically compress these files by as much as 80-90%, dramatically reducing network congestion, connect times, and mailbox bloat. Since attachments are also a primary vector for virus transmission, ZipOut can be set to restrict or block the transfer of specific file types. ZipOut can also perform simple or complex searches of your mailbox or Personal Folders files for all types of attachments, including files within zipped attachments and embedded messages.